A gravitational tug-of-war has warped the spiral shape of NGC 3169 and fragmented the dust lanes in its companion, NGC 3166.

This image from the Wide Field Imager on the MPG/ESO 2.2-meter telescope at the La Silla Observatory in Chile captures the pair of galaxies NGC 3169 (left) and NGC 3166 (right). These adjacent galaxies display some curious features, demonstrating that each member of the duo is close enough to feel the distorting gravitational influence of the other. The gravitational tug-of-war has warped the spiral shape of one galaxy, NGC 3169, and fragmented the dust lanes in its companion NGC 3166.
Photo by ESO/Igor Chekalin
By ESO, Garching, Germany
Published: April 21, 2011

The galaxies in this cosmic pairing, captured by the Wide Field Imager on the MPG/ESO 2.2-meter telescope at the La Silla Observatory in Chile, display some curious features, demonstrating that each member of the duo is close enough to feel the distorting gravitational influence of the other. The gravitational tug-of-war has warped the spiral shape of one galaxy, NGC 3169, and fragmented the dust lanes in its companion, NGC 3166. Meanwhile, a third, smaller galaxy to the lower right, NGC 3165, has a front-row seat to the gravitational twisting and pulling of its bigger neighbors.

This galactic grouping, found about 70 million light-years away in the constellation Sextans, was discovered by the English astronomer William Herschel in 1783. Modern astronomers have gauged the distance between NGC 3169 (left) and NGC 3166 (right) as a mere 50,000 light-years, a separation that is only about half the diameter of the Milky Way Galaxy. In such tight quarters, gravity can start to play havoc with galactic structure.

Spiral galaxies like NGC 3169 and NGC 3166 tend to have orderly swirls of stars and dust pinwheeling about their glowing centers. Close encounters with other massive objects can jumble this classic configuration, often serving as a disfiguring prelude to the merging of galaxies into one larger object. So far, the interactions of NGC 3169 and NGC 3166 have just lent a bit of character. NGC 3169’s arms, shining bright with big, young, blue stars, have been teased apart, and lots of luminous gas has been drawn out from its disk. In NGC 3166’s case, the dust lanes that also usually outline spiral arms are in disarray. Unlike its bluer counterpart, NGC 3166 is not forming many new stars.

NGC 3169 has another distinction: the faint yellow dot beaming through a veil of dark dust just to the left of and close to the galaxy’s center. This flash is the leftover of a supernova detected in 2003 and known accordingly as SN 2003cg. A supernova of this variety, classified as a type Ia, is thought to occur when a dense, hot star called a white dwarf — a remnant of medium-sized stars like our Sun — gravitationally sucks gas away from a nearby companion star. This added fuel eventually causes the whole star to explode in a runaway fusion reaction.

The new image presented here of a remarkable galactic dynamic duo is based on data selected by Igor Chekalin for the European Southern Observatory’s Hidden Treasures 2010 astrophotography competition. Chekalin won the first overall prize and this image received the second-highest ranking of the nearly 100 contest entries.

When Galaxies Collide!

NGC 2623: Galaxy Merger from Hubble
Credit: NASA, ESA and A. Evans (Stony Brook).

Where do stars form when galaxies collide? To help find out, astronomers imaged the nearby galaxy merger NGC 2623 in high resolution with the Hubble Space Telescope in 2007. Analysis of this Hubble image and images of NGC 2623 in infrared light by the Spitzer Space Telescope, in X-ray light by XMM-Newton, and in ultraviolet light by GALEX, indicate that two originally spiral galaxies appear now to be greatly convolved and that their cores have unified into one active galactic nucleus (AGN). Star formation continues around this core near the above image center, along the stretched out tidal tails visible on either side, and perhaps surprisingly, in an off-nuclear region on the upper left where clusters of bright blue stars appear. Galaxy collisions can take hundreds of millions of years and take several gravitationally destructive passes. NGC 2623, also known as Arp 243, spans about 50,000 light years and lies about 250 million light years away toward the constellation of the Crab (Cancer). Reconstructing the original galaxies and how galaxy mergers happen is often challenging, sometimes impossible, but generally important to understanding how our universe evolved.

Date:14th November,2010

Galaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxy interaction. Although galaxy mergers do not involve stars or star systems actually colliding, due to the vast distances between stars in most circumstances, the gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. There are some generally accepted results, however:

1. When one of the galaxies is significantly larger than the other, the larger will often "eat" the smaller, absorbing most of its gas and stars with little other major effect on the larger galaxy. Our home galaxy, the Milky Way, is thought to be currently absorbing smaller galaxies in this fashion, such as the Canis Major Dwarf Galaxy, and possibly the Magellanic Clouds. The Virgo Stellar Stream is thought to be the remains of a dwarf galaxy that has been mostly merged with the Milky Way.

2. If two spiral galaxies that are approximately the same size collide at appropriate angles and speeds, they will likely merge in a fashion that drives away much of the dust and gas through a variety of feedback mechanisms that often include a stage in which there are active galactic nuclei. This is thought to be the driving force behind many quasars. The end result is an elliptical galaxy, and many astronomers hypothesize that this is the primary mechanism that creates ellipticals.

Note that the Milky Way and the Andromeda Galaxy will probably collide in about 4.5 billion years. If these galaxies merged, the result would quite possibly be an elliptical galaxy as described above.

One of the largest galaxy mergers ever observed consisted of four elliptical galaxies in the cluster CL0958+4702. It may form one of the largest galaxies in the Universe.

Galaxy mergers can be simulated in computers, to learn more about galaxy formation. Galaxy pairs initially of any morphological type can be followed, taking into account all gravitational forces, and also the hydrodynamics and dissipation of the interstellar gas, the star formation out of the gas, and the energy and mass released back in the interstellar medium by supernovae.

Simulating the Fate of Our Milky Way

Our Future: A simulation of what might happen when the Andromeda Galaxy hits ours shows tidal forces of gravity creating long plumes of material. The central regions will relatively quickly fall back together and merge into a single remnant galaxy.

When cars collide, it’s an accident. When galaxies collide, it’s Nature at work. Many astronomers believe such crashes are part of the natural evolution in the lives of galaxies and galaxy clusters, now scientists are blending scientific research and high-powered computer visual effects into vivid models of how they occur.

Astrophysicist Frank Summers, of the Space Telescope Science Institute (STScI), produced a simulated galaxy collision, using a combination of computer modeling research and the same special effects software used to make computer-generated movies.

The resulting animation shows a crash between two large spiral galaxies roughly the size of our own Milky Way and its larger neighbor Andromeda, themselves slated to collide in a few billion years.

"What makes me happy about this visualization is that it’s a presentation of accurate science to the public," Summers told. "It allows for a fluid process of education and the ability to create mental models of the universe."

Rip and tear

In his animation, Summers shows two galaxies at different planes of position, then documents their collision at a rate of about 10 million years per second. The entire sequence covers some 500 million years.

As the galaxies approach each other, they keep their spiral shapes up to the point of impact, where so-called "tidal forces" of gravity result in the formation of long plumes of stars, gas and dust called tidal tails. The centers of each galaxy then merge into one remnant core. The scenario is a likely preview for the expected interaction between the Milky Way and Andromeda galaxies, which some astronomers have already mapped out with computers models.

Summers used research data produced by astronomy professors and galactic modelers Chris Mihos, of Case Western Reserve University, and Lars Hernquist, of Harvard University. The researchers used a supercomputer to depict the collision in a project for the National Air and Space Museum's newly renovated Einstein Planetarium.

Dark matter

Astronomers who study galaxy structure spend most of their time modeling the effects of dark matter, that ubiquitous but unseen stuff that makes up most of a galaxy’s mass, Summers said. Only between 10 percent and 30 percent of a galaxy’s mass is visible, so astronomers study its rotation to determine the dark matter content, material that must be there based on known gravitational effects. They also look at how clusters of galaxies appear glued together by gravity.

By simulating collisions between galaxies, theorists can study the structures of galaxies and the architecture behind galactic clusters without having to wait the millennia it takes for such crashes to occur.

In the early days of the universe, the rate of collisions was about 10 to 100 times higher, simply because things were closer together. Although individual stars may not physically hit each other during a collision -- the space between them is still vast -- the gravitational effects of the encounter are enough to twist and distort galaxies beyond recognition.

Colliding spiral galaxies can become one elliptical galaxy, for example, which is the likely destiny for our Milky Way, astronomers say.

Galaxy building

"Mergers and interactions between galaxies are an essential part of their dynamical evolution," said John Dubinksi, an astronomy professor at University of Toronto who has modeled the eventual clash between the Milky Way and Andromeda galaxies. "The elliptical galaxies which represent around 10 percent of the galaxy population are most likely the product of a merger of two or more galaxies of nearly equal mass."

Galaxy collisions also contribute to star formation, as clouds of gas heat up and coalesce during the encounter. Observations from the Chandra X-ray Observatory suggest they may even contribute to the development of supermassive black holes.

In galaxy clusters, Dubinski said in an e-mail interview, elliptical galaxies outnumber spiral or irregular varieties. The clusters also often have a giant elliptical galaxy -- a product of many spiral galaxies merging together when the cluster first formed -- at their center.

Where we're headed

Most scientists agree that the Milky Way will cross paths with the Andromeda galaxy in about three billion years. Both galaxies are now spiral in shape, though Andromeda is about twice as large as the Milky Way.

The galaxies are separated by about 2.2 million light years (one light-year is about 6 trillion miles, or 10 trillion kilometers). That gap is closing at about 310,000 miles per hour (500,000 kph).

While a collision appears inevitable, astronomers admit that the sideways motion of Andromeda -- the galaxy’s speed perpendicular to its forward path toward the Milky Way -- could affect the encounter’s timing, but it has yet to be measured precisely. Dubinksi used an estimate of 12.4 miles per second (20 km per second) for his collision model.

"Even if the galaxies have a wider passage on the first pass, if they are on a bound orbit they are destined to merge eventually," Dubinski said. "If not on the first flyby, then within the second or third pass over the next 10 billion years, he added.

The clincher is gravity. Even if there’s enough space between the Milky Way and Andromeda to simply brush past each other at spiral arm’s length, their mutual gravity will ultimately win out, drawing the two galaxies together on successive flybys.Dubinski hopes to refine his model of the collision between the Milky Way and Andromeda galaxies in the future by modeling a system of about a trillion or so particles to match the number of stars in the two galaxies. But with the current growth in computer memory and speed, such computations won’t be possible for about 10 years, he said.

The Mice

Meanwhile, another pair of researchers has taken a pair of interacting galaxies called the Mice and worked backward to simulate what they figure has already taken place. The Mice, recently photographed by the Hubble Space Telescope's new camera, represent a collision in progress that could be very much like the pending crash of our own galaxy into Andromeda.

Joshua Barnes of the University of Hawaii worked with John Hibbard, now at the National Radio Astronomy Observatory, to animate a past that might have led to the present-day Mice.

Their computer animation shows two pinwheel galaxies falling together, swerving as they pass, and flinging out long tails of stars. At present the two galaxies have made one pass, and are coming back for a second and closer encounter. Eventually they will coalesce into a single galaxy, whose possibilities the simulation projects.

"Simulating colliding galaxies is a bit like investigating a car crash," Barnes says. "Suppose you had no witnesses, just a couple of wrecked cars. You might try different test crashes, varying things like speed and angle of impact, until you found a way to get the same damage as the original collision. That's basically what we did."